The role of GRASP55 in Golgi fragmentation and entry of cells into mitosis

RNA scaffolds the Golgi ribbon by forming condensates with GM130

The mammalian Golgi is composed of stacks that are laterally connected into a continuous ribbon-like structure. The integrity and function of the ribbon is disrupted under stress conditions, but the molecular mechanisms remain unclear. Here we show that the ribbon is maintained by biomolecular condensates of RNA and the Golgi matrix protein GM130 (GOLGA2). We identify GM130 as a membrane-bound RNA-binding protein, which directly recruits RNA and associated RNA-binding proteins to the Golgi membrane. Acute degradation of RNA or GM130 in cells disrupts the ribbon. Under stress conditions, RNA dissociates from GM130 and the ribbon is disjointed, but after the cells recover from stress the ribbon is restored. When overexpressed in cells, GM130 forms RNA-dependent liquid-like condensates. GM130 contains an intrinsically disordered domain at its amino terminus, which binds RNA to induce liquid-liquid phase separation. These co-condensates are sufficient to link purified Golgi membranes, reconstructing lateral linking of stacks into a ribbon-like structure. Together, these studies show that RNA acts as a structural biopolymer that together with GM130 maintains the integrity of the Golgi ribbon.

Ceramide-1-phosphate is a regulator of Golgi structure and is co-opted by the obligate intracellular bacterial pathogen Anaplasma phagocytophilum

Many intracellular pathogens structurally disrupt the Golgi apparatus as an evolutionarily conserved promicrobial strategy. Yet, the host factors and signaling processes involved are often poorly understood, particularly for Anaplasma phagocytophilum, the agent of human granulocytic anaplasmosis. We found that A. phagocytophilum elevated cellular levels of the bioactive sphingolipid, ceramide-1-phosphate (C1P), to promote Golgi fragmentation that enables bacterial proliferation, conversion from its non-infectious to infectious form, and productive infection. A. phagocytophilum poorly infected mice deficient in ceramide kinase, the Golgi-localized enzyme responsible for C1P biosynthesis. C1P regulated Golgi morphology via activation of a PKCα/Cdc42/JNK signaling axis that culminates in phosphorylation of Golgi structural proteins, GRASP55 and GRASP65. siRNA-mediated depletion of Cdc42 blocked A. phagocytophilum from altering Golgi morphology, which impaired anterograde trafficking of trans-Golgi vesicles into and maturation of the pathogen-occupied vacuole. Cells overexpressing phosphorylation-resistant versions of GRASP55 and GRASP65 presented with suppressed C1P- and A. phagocytophilum-induced Golgi fragmentation and poorly supported infection by the bacterium. By studying A. phagocytophilum, we identify C1P as a regulator of Golgi structure and a host factor that is relevant to disease progression associated with Golgi fragmentation.IMPORTANCECeramide-1-phosphate (C1P), a bioactive sphingolipid that regulates diverse processes vital to mammalian physiology, is linked to disease states such as cancer, inflammation, and wound healing. By studying the obligate intracellular bacterium Anaplasma phagocytophilum, we discovered that C1P is a major regulator of Golgi morphology. A. phagocytophilum elevated C1P levels to induce signaling events that promote Golgi fragmentation and increase vesicular traffic into the pathogen-occupied vacuole that the bacterium parasitizes. As several intracellular microbial pathogens destabilize the Golgi to drive their infection cycles and changes in Golgi morphology is also linked to cancer and neurodegenerative disorder progression, this study identifies C1P as a potential broad-spectrum therapeutic target for infectious and non-infectious diseases.

FvKex2 is required for development, virulence, and mycotoxin production in Fusarium verticillioides

Fusarium verticillioides is one of the most important fungal pathogens causing maize ear and stalk rots, thereby undermining global food security. Infected seeds are usually unhealthy for consumption due to contamination with fumonisin B1 (FB1) mycotoxin produced by the fungus as a virulence factor. Unveiling the molecular factors that determine fungal development and pathogenesis will help in the control and management of the diseases. Kex2 is a kexin-like Golgi-resident proprotein convertase that is involved in the activation of some important proproteins. Herein, we identified and functionally characterized FvKex2 in relation to F. verticillioides development and virulence by bioinformatics and functional genomics approaches. We found that FvKex2 is required for the fungal normal vegetative growth, because the growth of the ∆Fvkex2 mutant was significantly reduced on culture media compared to the wild-type and complemented strains. The mutant also produced very few conidia with morphologically abnormal shapes when compared with those from the wild type. However, the kexin-like protein was dispensable for the male role in sexual reproduction in F. verticillioides. In contrast, pathogenicity was nearly abolished on wounded maize stalks and sugarcane leaves in the absence of FvKEX2 gene, suggesting an essential role of Fvkex2 in the virulence of F. verticillioides. Furthermore, high-performance liquid chromatography analysis revealed that the ∆Fvkex2 mutant produced a significantly lower level of FB1 mycotoxin compared to the wild-type and complemented strains, consistent with the loss of virulence observed in the mutant. Taken together, our results indicate that FvKex2 is critical for vegetative growth, FB1 biosynthesis, and virulence, but dispensable for sexual reproduction in F. verticillioides. The study presents the kexin-like protein as a potential drug target for the management of the devastating maize ear and stalk rot diseases. Further studies should aim at uncovering the link between FvKex2 activity and FB1 biosynthesis genes. KEY POINTS: •The kexin-like protein FvKex2 contributes significantly to the vegetative growth of Fusarium verticillioides. •The conserved protein is required for fungal conidiation and conidial morphology, but dispensable for sexual reproduction. •Deletion of FvKEX2 greatly attenuates the virulence and mycotoxin production potential of F. verticillioides.

The crescent-like Golgi ribbon is shaped by the Ajuba/PRMT5/Aurora-A complex-modified HURP

BACKGROUND

Golgi apparatus (GA) is assembled as a crescent-like ribbon in mammalian cells under immunofluorescence microscope without knowing the shaping mechanisms. It is estimated that roughly 1/5 of the genes encoding kinases or phosphatases in human genome participate in the assembly of Golgi ribbon, reflecting protein modifications play major roles in building Golgi ribbon.

METHODS

To explore how Golgi ribbon is shaped as a crescent-like structure under the guidance of protein modifications, we identified a protein complex containing the scaffold proteins Ajuba, two known GA regulators including the protein kinase Aurora-A and the protein arginine methyltransferase PRMT5, and the common substrate of Aurora-A and PRMT5, HURP. Mutual modifications and activation of PRMT5 and Aurora-A in the complex leads to methylation and in turn phosphorylation of HURP, thereby producing HURP p725. The HURP p725 localizes to GA vicinity and its distribution pattern looks like GA morphology. Correlation study of the HURP p725 statuses and GA structure, site-directed mutagenesis and knockdown-rescue experiments were employed to identify the modified HURP as a key regulator assembling GA as a crescent ribbon.

RESULTS

The cells containing no or extended distribution of HURP p725 have dispersed GA membranes or longer GA. Knockdown of HURP fragmentized GA and HURP wild type could, while its phosphorylation deficiency mutant 725A could not, restore crescent Golgi ribbon in HURP depleted cells, collectively indicating a crescent GA-constructing activity of HURP p725. HURP p725 is transported, by GA membrane-associated ARF1, Dynein and its cargo adaptor Golgin-160, to cell center where HURP p725 forms crescent fibers, binds and stabilizes Golgi assembly factors (GAFs) including TRIP11, GRASP65 and GM130, thereby dictating the formation of crescent Golgi ribbon at nuclear periphery.

CONCLUSIONS

The Ajuba/PRMT5/Aurora-A complex integrates the signals of protein methylation and phosphorylation to HURP, and the HURP p725 organizes GA by stabilizing and recruiting GAFs to its crescent-like structure, therefore shaping GA as a crescent ribbon. Therefore, the HURP p725 fiber serves a template to construct GA according to its shape. Video Abstract.

Auxin-inducible degron system: an efficient protein degradation tool to study protein function

Targeted protein degradation, with its rapid protein depletion kinetics, allows the measurement of acute changes in the cell. The auxin-inducible degron (AID) system, rapidly degrades AID-tagged proteins only in the presence of auxin. The AID system being inducible makes the study of essential genes and dynamic processes like cell differentiation, cell cycle and genome organization feasible. The AID degradation system has been adapted to yeast, protozoans, C. elegans, Drosophila, zebrafish, mouse and mammalian cell lines. Using the AID system, researchers have unveiled novel functions for essential proteins at developmental stages that were previously difficult to investigate due to early lethality. This comprehensive review discusses the development, advancements, applications and drawbacks of the AID system and compares it with other available protein degradation systems.

Impact of interorganelle coordination between the conventional early secretory pathway and autophagy in cellular homeostasis and stress response

The conventional early secretory pathway and autophagy are two essential interconnected cellular processes that are crucial for maintaining cellular homeostasis. The conventional secretory pathway is an anabolic cellular process synthesizing and delivering proteins to distinct locations, including different organelles, the plasma membrane, and the extracellular media. On the other hand, autophagy is a catabolic cellular process that engulfs damaged organelles and aberrant cytosolic constituents into the double autophagosome membrane. After fusion with the lysosome and autolysosome formation, this process triggers digestion and recycling. A growing list of evidence indicates that these anabolic and catabolic processes are mutually regulated. While knowledge about the molecular actors involved in the coordination and functional cooperation between these two processes has increased over time, the mechanisms are still poorly understood. This review article summarized and discussed the most relevant evidence about the key molecular players implicated in the interorganelle crosstalk between the early secretory pathway and autophagy under normal and stressful conditions.

GRASP55 restricts early-stage autophagy and regulates spatial organization of the early secretory network

There is great interest in understanding the cellular mechanisms controlling autophagy, a tightly regulated catabolic and stress-response pathway. Prior work has uncovered links between autophagy and the Golgi reassembly stacking protein of 55 kDa (GRASP55), but their precise interrelationship remains unclear. Intriguingly, both autophagy and GRASP55 have been functionally and spatially linked to the endoplasmic reticulum (ER)­­-Golgi interface, broaching this compartment as a site where GRASP55 and autophagy may intersect. Here, we uncover that loss of GRASP55 enhances LC3 puncta formation, indicating that GRASP55 restricts autophagosome formation. Additionally, using proximity-dependent biotinylation, we identify a GRASP55 proximal interactome highly associated with the ER-Golgi interface. Both nutrient starvation and loss of GRASP55 are associated with coalescence of early secretory pathway markers. In light of these findings, we propose that GRASP55 regulates spatial organization of the ER-Golgi interface, which suppresses early autophagosome formation.

Summary: The Golgi protein GRASP55 restricts early-stage autophagy and regulates spatial organization of the early secretory network. We also identify a GRASP55 proximal interactome enriched at the ER-Golgi interface.

Gβγ regulates mitotic Golgi fragmentation and G2/M cell cycle progression

Heterotrimeric G proteins (αβγ) function at the cytoplasmic surface of a cell’s plasma membrane to transduce extracellular signals into cellular responses. However, numerous studies indicate that G proteins also play noncanonical roles at unique intracellular locations. Previous work has established that G protein βγ subunits (Gβγ) regulate a signaling pathway on the cytoplasmic surface of Golgi membranes that controls the exit of select protein cargo. Now, we demonstrate a novel role for Gβγ in regulating mitotic Golgi fragmentation, a key checkpoint of the cell cycle that occurs in the late G2 phase. We show that small interfering RNA–mediated depletion of Gβ1 and Gβ2 in synchronized cells causes a decrease in the number of cells with fragmented Golgi in late G2 and a delay of entry into mitosis and progression through G2/M. We also demonstrate that during G2/M Gβγ acts upstream of protein kinase D and regulates the phosphorylation of the Golgi structural protein GRASP55. Expression of Golgi-targeted GRK2ct, a Gβγ-sequestering protein used to inhibit Gβγ signaling, also causes a decrease in Golgi fragmentation and a delay in mitotic progression. These results highlight a novel role for Gβγ in regulation of Golgi structure.

Rapid degradation of GRASP55 and GRASP65 reveals their immediate impact on the Golgi structure

GRASP55 and GRASP65 have been implicated in stacking of Golgi cisternae and lateral linking of stacks within the Golgi ribbon. However, RNAi or gene knockout approaches to dissect their respective roles have often resulted in conflicting conclusions. Here, we gene-edited GRASP55 and/or GRASP65 with a degron tag in human fibroblasts, allowing for induced rapid degradation by the proteasome. We show that acute depletion of either GRASP55 or GRASP65 does not affect the Golgi ribbon, while chronic degradation of GRASP55 disrupts lateral connectivity of the ribbon. Acute double depletion of both GRASPs coincides with the loss of the vesicle tethering proteins GM130, p115, and Golgin-45 from the Golgi and compromises ribbon linking. Furthermore, GRASP55 and/or GRASP65 is not required for maintaining stacks or de novo assembly of stacked cisternae at the end of mitosis. These results demonstrate that both GRASPs are dispensable for Golgi stacking but are involved in maintaining the integrity of the Golgi ribbon together with GM130 and Golgin-45.

The function of GORASPs in Golgi apparatus organization in vivo

In vitro experiments have shown that GRASP65 (GORASP1) and GRASP55 (GORASP2) proteins function in stacking Golgi cisternae. However, in vivo depletion of GORASPs in metazoans has given equivocal results. We have generated a mouse lacking both GORASPs and find that Golgi cisternae remained stacked. However, the stacks are disconnected laterally from each other, and the cisternal cross-sectional diameters are significantly reduced compared with their normal counterparts. These data support earlier findings on the role of GORASPs in linking stacks, and we suggest that unlinking of stacks likely affects dynamic control of COPI budding and vesicle fusion at the rims. The net result is that cisternal cores remain stacked, but cisternal diameter is reduced by rim consumption.

Nonredundant Roles of GRASP55 and GRASP65 in the Golgi Apparatus and Beyond

It has been demonstrated that two Golgi stacking proteins, GRASP55 and GRASP65, self-interact to form trans-oligomers that tether adjacent Golgi membranes into stacks and ribbons in mammalian cells. This ensures proper functioning of the Golgi apparatus in protein trafficking and processing. More recently, GRASP proteins have drawn extensive attention from researchers due to their diverse and essential roles in and out of the Golgi in different organisms. In this review, we summarize their established roles in Golgi structure formation and function under physiological conditions. We then highlight the emerging and divergent roles for individual GRASP proteins, focusing on GRASP65 in cell migration and apoptosis and GRASP55 in unconventional protein secretion and autophagy under stress or pathological conditions.

The Golgi ribbon: mechanisms of maintenance and disassembly during the cell cycle

The Golgi complex (GC) has an essential role in the processing and sorting of proteins and lipids. The GC of mammalian cells is composed of stacks of cisternae connected by membranous tubules to create a continuous network, the Golgi ribbon, whose maintenance requires several core and accessory proteins. Despite this complex structural organization, the Golgi apparatus is highly dynamic, and this property becomes particularly evident during mitosis, when the ribbon undergoes a multistep disassembly process that allows its correct partitioning and inheritance by the daughter cells. Importantly, alterations of the Golgi structure are associated with a variety of physiological and pathological conditions. Here, we review the core mechanisms and signaling pathways involved in both the maintenance and disassembly of the Golgi ribbon, and we also report on the signaling pathways that connect the disassembly of the Golgi ribbon to mitotic entry and progression.

SIRT2 deacetylates GRASP55 to facilitate post-mitotic Golgi assembly

Sirtuin 2 (SIRT2) is an NAD-dependent sirtuin deacetylase that regulates microtubule and chromatin dynamics, gene expression and cell cycle progression, as well as nuclear envelope reassembly. Recent proteomic analyses have identified Golgi proteins as SIRT2 interactors, indicating that SIRT2 may also play a role in Golgi structure formation. Here, we show that SIRT2 depletion causes Golgi fragmentation and impairs Golgi reassembly at the end of mitosis. SIRT2 interacts with the Golgi reassembly stacking protein GRASP55 (also known as GORASP2) in mitosis when GRASP55 is highly acetylated on K50. Expression of wild-type and the K50R acetylation-deficient mutant of GRASP55, but not the K50Q acetylation-mimetic mutant, in GRASP55 and GRASP65 (also known as GORASP1) double-knockout cells, rescued the Golgi structure and post-mitotic Golgi reassembly. Acetylation-deficient GRASP55 exhibited a higher self-interaction efficiency, a property required for Golgi structure formation. These results demonstrate that SIRT2 regulates Golgi structure by modulating GRASP55 acetylation levels.

A New Look at the Functional Organization of the Golgi Ribbon

A characteristic feature of vertebrate cells is a Golgi ribbon consisting of multiple cisternal stacks connected into a single-copy organelle next to the centrosome. Despite numerous studies, the mechanisms that link the stacks together and the functional significance of ribbon formation remain poorly understood. Nevertheless, these questions are of considerable interest, since there is increasing evidence that Golgi fragmentation – the unlinking of the stacks in the ribbon – is intimately connected not only to normal physiological processes, such as cell division and migration, but also to pathological states, including neurodegeneration and cancer. Challenging a commonly held view that ribbon architecture involves the formation of homotypic tubular bridges between the Golgi stacks, we present an alternative model, based on direct interaction between the biosynthetic (pre-Golgi) and endocytic (post-Golgi) membrane networks and their connection with the centrosome. We propose that the central domains of these permanent pre- and post-Golgi networks function together in the biogenesis and maintenance of the more transient Golgi stacks, and thereby establish “linker compartments” that dynamically join the stacks together. This model provides insight into the reversible fragmentation of the Golgi ribbon that takes place in dividing and migrating cells and its regulation along a cell surface – Golgi – centrosome axis. Moreover, it helps to understand transport pathways that either traverse or bypass the Golgi stacks and the positioning of the Golgi apparatus in differentiated neuronal, epithelial, and muscle cells.

GORASP2/GRASP55 collaborates with the PtdIns3K UVRAG complex to facilitate autophagosome-lysosome fusion

It has been indicated that the Golgi apparatus contributes to autophagy, but how it is involved in autophagosome formation and maturation is not well understood. Here we show that amino acid starvation causes trans-Golgi derived membrane fragments to colocalize with autophagosomes. Depletion of the Golgi stacking protein GORASP2/GRASP55, but not GORASP1/GRASP65, increases both MAP1LC3 (LC3)-II and SQSTM1/p62 levels. We demonstrate that GORASP2 facilitates autophagosome-lysosome fusion by physically linking autophagosomes and lysosomes through the interactions with LC3 on autophagosomes and LAMP2 on late endosomes/lysosomes. Furthermore, we provide evidence that GORASP2 interacts with BECN1 to facilitate the assembly and membrane association of the phosphatidylinositol 3-kinase (PtdIns3K) UVRAG complex. These findings indicate that GORASP2 plays an important role in autophagosome maturation during amino acid starvation. Abbreviations: ATG14: autophagy related 14; BafA1: bafilomycin A₁; BSA: bovine serum albumin; CQ: chloroquine; EBSS: earle's balanced salt solution; EM: electron microscopy; EEA1: early endosome antigen 1; GFP: green fluorescent protein; GORASP1/GRASP65: golgi reassembly stacking protein 1; GORASP2/GRASP55: golgi reassembly stacking protein 2; LAMP1: lysosomal-associated membrane protein 1; LAMP2: lysosomal-associated membrane protein 2; MAP1LC3: microtubule associated protein 1 light chain 3; MTOR: mechanistic target of rapamycin kinase; PBS: phosphate-buffered saline; PtdIns3K: phosphatidylinositol 3-kinase; PtdIns3P: phosphatidylinositol 3-phosphate; PK: protease K; PNS: post-nuclear supernatant; RFP: red fluorescent protein; SD: standard deviation; TGN: trans-Golgi network; UVRAG: UV radiation resistance associated.

GRASP depletion-mediated Golgi destruction decreases cell adhesion and migration via the reduction of α5β1 integrin

The Golgi apparatus is a membrane-bound organelle that serves as the center for trafficking and processing of proteins and lipids. To perform these functions, the Golgi forms a multilayer stacked structure held by GRASP55 and GRASP65 trans-oligomers and perhaps their binding partners. Depletion of GRASP proteins disrupts Golgi stack formation and impairs critical functions of the Golgi, such as accurate protein glycosylation and sorting. However, how Golgi destruction affects other cellular activities is so far unknown. Here, we report that depletion of GRASP proteins reduces cell attachment and migration. Interestingly, GRASP depletion reduces the protein level of α5β1 integrin, the major cell adhesion molecule at the surface of HeLa and MDA-MB-231 cells, due to decreased integrin protein synthesis. GRASP depletion also increases cell growth and total protein synthesis. These new findings enrich our understanding on the role of the Golgi in cell physiology and provide a potential target for treating protein-trafficking disorders.

Golgi Structure and Function in Health, Stress, and Diseases

The Golgi apparatus is a central intracellular membrane-bound organelle with key functions in trafficking, processing, and sorting of newly synthesized membrane and secretory proteins and lipids. To best perform these functions, Golgi membranes form a unique stacked structure. The Golgi structure is dynamic but tightly regulated; it undergoes rapid disassembly and reassembly during the cell cycle of mammalian cells and is disrupted under certain stress and pathological conditions. In the past decade, significant amount of effort has been made to reveal the molecular mechanisms that regulate the Golgi membrane architecture and function. Here we review the major discoveries in the mechanisms of Golgi structure formation, regulation, and alteration in relation to its functions in physiological and pathological conditions to further our understanding of Golgi structure and function in health and diseases.

DjA1 maintains Golgi integrity via interaction with GRASP65

In mammalian cells, the Golgi reassembly stacking protein of 65 kDa (GRASP65) has been implicated in both Golgi stacking and ribbon linking by forming trans-oligomers. To better understand its function and regulation, we used biochemical methods to identify the DnaJ homolog subfamily A member 1 (DjA1) as a novel GRASP65-binding protein. In cells, depletion of DjA1 resulted in Golgi fragmentation, short and improperly aligned cisternae, and delayed Golgi reassembly after nocodazole washout. In vitro, immunodepletion of DjA1 from interphase cytosol reduced its activity to enhance GRASP65 oligomerization and Golgi membrane fusion, while adding purified DjA1 enhanced GRASP65 oligomerization. DjA1 is a cochaperone of Heat shock cognate 71-kDa protein (Hsc70), but the activity of DjA1 in Golgi structure formation is independent of its cochaperone activity or Hsc70, rather, through DjA1-GRASP65 interaction to promote GRASP65 oligomerization. Thus, DjA1 interacts with GRASP65 to enhance Golgi structure formation through the promotion of GRASP65 trans-oligomerization.

Golgi structure formation, function, and post-translational modifications in mammalian cells

The Golgi apparatus is a central membrane organelle for trafficking and post-translational modifications of proteins and lipids in cells. In mammalian cells, it is organized in the form of stacks of tightly aligned flattened cisternae, and dozens of stacks are often linked laterally into a ribbon-like structure located in the perinuclear region of the cell. Proper Golgi functionality requires an intact architecture, yet Golgi structure is dynamically regulated during the cell cycle and under disease conditions. In this review, we summarize our current understanding of the relationship between Golgi structure formation, function, and regulation, with focus on how post-translational modifications including phosphorylation and ubiquitination regulate Golgi structure and on how Golgi unstacking affects its functions, in particular, protein trafficking, glycosylation, and sorting in mammalian cells.

Mitotic inheritance of the Golgi complex and its role in cell division

The Golgi apparatus plays essential roles in the processing and sorting of proteins and lipids, but it can also act as a signalling hub and a microtubule-nucleation centre. The Golgi complex (GC) of mammalian cells is composed of stacks connected by tubular bridges to form a continuous membranous system. In spite of this structural complexity, the GC is highly dynamic, and this feature becomes particularly evident during mitosis, when the GC undergoes a multi-step disassembly process that allows its correct partitioning and inheritance by daughter cells. Strikingly, different steps of Golgi disassembly control mitotic entry and progression, indicating that cells actively monitor Golgi integrity during cell division. Here, we summarise the basic mechanisms and the molecular players that are involved in Golgi disassembly, focussing in particular on recent studies that have revealed the fundamental signalling pathways that connect Golgi inheritance to mitotic entry and progression.

Genetic, structural, and chemical insights into the dual function of GRASP55 in germ cell Golgi remodeling and JAM-C polarized localization during spermatogenesis

Spermatogenesis is a dynamic process that is regulated by adhesive interactions between germ and Sertoli cells. Germ cells express the Junctional Adhesion Molecule-C (JAM-C, encoded by Jam3), which localizes to germ/Sertoli cell contacts. JAM-C is involved in germ cell polarity and acrosome formation. Using a proteomic approach, we demonstrated that JAM-C interacted with the Golgi reassembly stacking protein of 55 kDa (GRASP55, encoded by Gorasp2) in developing germ cells. Generation and study of Gorasp2-/- mice revealed that knock-out mice suffered from spermatogenesis defects. Acrosome formation and polarized localization of JAM-C in spermatids were altered in Gorasp2-/- mice. In addition, Golgi morphology of spermatocytes was disturbed in Gorasp2-/- mice. Crystal structures of GRASP55 in complex with JAM-C or JAM-B revealed that GRASP55 interacted via PDZ-mediated interactions with JAMs and induced a conformational change in GRASP55 with respect of its free conformation. An in silico pharmacophore approach identified a chemical compound called Graspin that inhibited PDZ-mediated interactions of GRASP55 with JAMs. Treatment of mice with Graspin hampered the polarized localization of JAM-C in spermatids, induced the premature release of spermatids and affected the Golgi morphology of meiotic spermatocytes.

Golgi ribbon disassembly during mitosis, differentiation and disease progression

The Golgi apparatus is tightly integrated into the cellular system where it plays essential roles required for a variety of cellular processes. Its vital functions include not only processing and sorting of proteins and lipids, but also serving as a signaling hub and a microtubule-organizing center. Golgi stacks in mammalian cells are interconnected into a compact ribbon in the perinuclear region. However, the ribbon can undergo distinct disassembly processes that reflect the cellular state or environmental demands and stress. For instance, its most dramatic change takes place in mitosis when the ribbon is efficiently disassembled into vesicles through a combination of ribbon unlinking, cisternal unstacking and vesiculation. Furthermore, the ribbon can also be detached and positioned at specific cellular locations to gain additional functionalities during differentiation, or fragmented to different degrees along disease progression or upon cell death. Here, we describe the major morphological alterations of Golgi ribbon disassembly under physiological and pathological conditions and discuss the underlying mechanisms that drive these changes.

Alterations of Golgi organization in Alzheimer's disease: A cause or a consequence?

The Golgi apparatus is a central organelle of the secretory pathway involved in the post-translational modification and sorting of lipids and proteins. In mammalian cells, the Golgi apparatus is composed of stacks of cisternae organized in polarized manner, which are interconnected by membrane tubules to constitute the Golgi ribbon, located in the proximity of the centrosome. Besides the processing and transport of cargo, the Golgi complex is actively involved in the regulation of mitotic entry, cytoskeleton organization and dynamics, calcium homeostasis, and apoptosis, representing a signalling platform for the control of several cellular functions, including signalling initiated by receptors located at the plasma membrane. Alterations of the conventional Golgi organization are associated to many disorders, such as cancer or different neurodegenerative diseases. In this review, we examine the functional implications of modifications of Golgi structure in neurodegenerative disorders, with a focus on the role of Golgi fragmentation in the development of Alzheimer's disease. The comprehension of the mechanism that induces Golgi fragmentation and of its downstream effects on neuronal function have the potential to contribute to the development of more effective therapies to treat or prevent some of these disorders.

Phosphorylation of Golgi Peripheral Membrane Protein Grasp65 Is an Integral Step in the Formation of the Human Cytomegalovirus Cytoplasmic Assembly Compartment

Human cytomegalovirus (HCMV) is the largest member of the Herpesviridae and represents a significant cause of disease. During virus replication, HCMV alters cellular functions to facilitate its replication, including significant reorganization of the secretory and endocytic pathways of the infected cell. A defining morphologic change of the infected cell is the formation of a membranous structure in the cytoplasm that is designated the virion assembly compartment (AC), which consists of virion structural proteins surrounded by cellular membranes. The loss of normal Golgi compartment morphology and its relocalization from a juxtanuclear ribbonlike structure to a series of concentric rings on the periphery of the AC represents a readily recognized reorganization of cellular membranes in the HCMV-infected cell. Although trafficking of viral proteins to this compartment is required for the assembly of infectious virions, the functional significance of the reorganization of intracellular membranes like the Golgi membranes into the AC in the assembly of infectious virus remains understudied. In this study, we determined that Golgi membrane ribbon fragmentation increased during the early cytoplasmic phase of virion assembly and that Golgi membrane fragmentation in infected cells was dependent on the phosphorylation of an integral cis-Golgi protein, Grasp65. Inhibition of Golgi membrane fragmentation and of its reorganization into the AC resulted in decreased production of infectious particles and alteration of the incorporation of an essential protein into the envelope of the mature virion. These results demonstrated the complexity of the virus-host cell interactions required for efficient assembly of this large DNA virus.

The human cytomegalovirus (HCMV)-induced reorganization of intracellular membranes that is required for the formation of the viral assembly compartment (AC) has been an area of study over the last 20 years. The significance of this virus-induced structure has been evinced by the results of several studies which showed that relocalization of viral proteins to the AC was required for efficient assembly of infectious virus. In this study, we have identified a mechanism for the fragmentation of the Golgi ribbon in the infected cell en route to AC morphogenesis. Identification of this fundamental process during HCMV replication allowed us to propose that the functional role of Golgi membrane reorganization during HCMV infection was the concentration of viral structural proteins and subviral structures into a single intracellular compartment in order to facilitate efficient protein-protein interactions and the virion protein trafficking required for the assembly of this large and structurally complex virus.

Glycosylation Quality Control by the Golgi Structure

Glycosylation is a ubiquitous modification that occurs on proteins and lipids in all living cells. Consistent with their high complexity, glycans play crucial biological roles in protein quality control and recognition events. Asparagine-linked protein N-glycosylation, the most complex glycosylation, initiates in the endoplasmic reticulum and matures in the Golgi apparatus. This process not only requires an accurate distribution of processing machineries, such as glycosyltransferases, glycosidases, and nucleotide sugar transporters, but also needs an efficient and well-organized factory that is responsible for the fidelity and quality control of sugar chain processing. In addition, accurate glycosylation must occur in coordination with protein trafficking and sorting. These activities are carried out by the Golgi apparatus, a membrane organelle in the center of the secretory pathway. To accomplish these tasks, the Golgi has developed into a unique stacked structure of closely aligned, flattened cisternae in which Golgi enzymes reside; in mammalian cells, dozens of Golgi stacks are often laterally linked into a ribbon-like structure. Here, we review our current knowledge of how the Golgi structure is formed and why its formation is required for accurate glycosylation, with the focus on how the Golgi stacking factors GRASP55 and GRASP65 generate the Golgi structure and how the conserved oligomeric Golgi complex maintains Golgi enzymes in different Golgi subcompartments by retrograde protein trafficking.

GRASP: A Multitasking Tether

Originally identified as Golgi stacking factors in vitro, the Golgi reassembly stacking protein (GRASP) family has been shown to act as membrane tethers with multiple cellular roles. As an update to previous comprehensive reviews of the GRASP family (Giuliani et al., 2011; Vinke et al., 2011; Jarvela and Linstedt, 2012), we outline here the latest findings concerning their diverse roles. New insights into the mechanics of GRASP-mediated tethering come from recent crystal structures. The models of how GRASP65 and GRASP55 tether membranes relate directly to their role in Golgi ribbon formation in mammalian cells and the unlinking of the ribbon at the onset of mitosis. However, it is also clear that GRASPs act outside the Golgi with roles at the ER and ER exit sites (ERES). Furthermore, the proteins of this family display other roles upon cellular stress, especially in mediating unconventional secretion of both transmembrane proteins (Golgi bypass) and cytoplasmic proteins (through secretory autophagosomes).

GRASPs in Golgi Structure and Function

The Golgi apparatus is a central intracellular membrane organelle for trafficking and modification of proteins and lipids. Its basic structure is a stack of tightly aligned flat cisternae. In mammalian cells, dozens of stacks are concentrated in the pericentriolar region and laterally connected to form a ribbon. Despite extensive research in the last decades, how this unique structure is formed and why its formation is important for proper Golgi functioning remain largely unknown. The Golgi ReAssembly Stacking Proteins, GRASP65, and GRASP55, are so far the only proteins shown to function in Golgi stacking. They are peripheral membrane proteins on the cytoplasmic face of the Golgi cisternae that form trans-oligomers through their N-terminal GRASP domain, and thereby function as the “glue” to stick adjacent cisternae together into a stack and to link Golgi stacks into a ribbon. Depletion of GRASPs in cells disrupts the Golgi structure and results in accelerated protein trafficking and defective glycosylation. In this minireview we summarize our current knowledge on how GRASPs function in Golgi structure formation and discuss why Golgi structure formation is important for its function.

Mechanisms and Regulation of the Mitotic Inheritance of the Golgi Complex

In mammalian cells, the Golgi complex is structured in the form of a continuous membranous system composed of stacks connected by tubular bridges: the "Golgi ribbon." At the onset of mitosis, the Golgi complex undergoes a multi-step fragmentation process that is required for its correct partition into the dividing cells. Importantly, inhibition of Golgi disassembly results in cell-cycle arrest at the G2 stage, which indicates that accurate inheritance of the Golgi complex is monitored by a "Golgi mitotic checkpoint." Moreover, mitotic Golgi disassembly correlates with the release of a set of Golgi-localized proteins that acquire specific functions during mitosis, such as mitotic spindle formation and regulation of the spindle checkpoint. Most of these events are regulated by small GTPases of the Arf and Rab families. Here, we review recent studies that are revealing the fundamental mechanisms, the molecular players, and the biological significance of mitotic inheritance of the Golgi complex in mammalian cells. We also briefly comment on how Golgi partitioning is coordinated with mitotic progression.

CaMKKβ-AMPKα2 signaling contributes to mitotic Golgi fragmentation and the G2/M transition in mammalian cells

Before a cell enters mitosis, the Golgi apparatus undergoes extensive fragmentation. This is required for the correct partitioning of the Golgi apparatus into daughter cells, and inhibition of this process leads to cell cycle arrest in G2 phase. AMP-activated protein kinase (AMPK) plays critical roles in regulating growth and reprogramming metabolism. Recent studies have suggested that AMPK promotes mitotic progression and Golgi disassembly, and that this seems independent of the cellular energy status. However, the molecular mechanism underlying these events is not well understood. Here, we show that both treatment with compound C and depletion of AMPKα2 (but not AMPKα1) delays the G2/M transition in synchronized HeLa cells, as evidenced by flow cytometry and mitotic index analysis. Furthermore, knockdown of AMPKα2 specifically delays further fragmentation of isolated Golgi stacks. Interestingly, pAMPKα(Thr172) signals transiently appear in the perinuclear region of late G2/early prophase cells, partially co-localizing with the Golgi matrix protein, GM-130. These Golgi pAMPKα(Thr172) signals were also specifically abolished by AMPKα2 knockdown, indicating specific spatio-temporal activation of AMPKα2 at Golgi complex during late G2/early prophases. We also found that the specific CaMKKβ inhibitor, STO-609, reduces the pAMPKα (Thr172) signals in the perinuclear region of G2 phase cells and delays mitotic Golgi fragmentation. Taken together, these data suggest that AMPKα2 is the major catalytic subunit of AMPKα which regulates Golgi fragmentation and G2/M transition, and that the CaMKKβ activates AMPKα2 during late G2 phase.

Mena-GRASP65 interaction couples actin polymerization to Golgi ribbon linking

GRASP65 plays a role in Golgi ribbon formation. Because the gaps between Golgi stacks are heterogeneous and large, it is possible that other proteins may help GRASP65 in ribbon linking. Mena is a novel GRASP65-binding protein that promotes actin elongation and enhances GRASP65 oligomerization to link Golgi stacks into a ribbon.

In mammalian cells, the Golgi reassembly stacking protein 65 (GRASP65) has been implicated in both Golgi stacking and ribbon linking by forming trans-oligomers through the N-terminal GRASP domain. Because the GRASP domain is globular and relatively small, but the gaps between stacks are large and heterogeneous, it remains puzzling how GRASP65 physically links Golgi stacks into a ribbon. To explore the possibility that other proteins may help GRASP65 in ribbon linking, we used biochemical methods and identified the actin elongation factor Mena as a novel GRASP65-binding protein. Mena is recruited onto the Golgi membranes through interaction with GRASP65. Depleting Mena or disrupting actin polymerization resulted in Golgi fragmentation. In cells, Mena and actin were required for Golgi ribbon formation after nocodazole washout; in vitro, Mena and microfilaments enhanced GRASP65 oligomerization and Golgi membrane fusion. Thus Mena interacts with GRASP65 to promote local actin polymerization, which facilitates Golgi ribbon linking.

Structural basis for the interaction between the Golgi reassembly-stacking protein GRASP65 and the Golgi matrix protein GM130

GM130 and GRASP65 are Golgi peripheral membrane proteins that play a key role in Golgi stacking and vesicle tethering. However, the molecular details of their interaction and their structural role as a functional unit remain unclear. Here, we present the crystal structure of the PDZ domains of GRASP65 in complex with the GM130 C-terminal peptide at 1.96-Å resolution. In contrast to previous findings proposing that GM130 interacts with GRASP65 at the PDZ2 domain only, our crystal structure of the complex indicates that GM130 binds to GRASP65 at two distinct sites concurrently and that both the PDZ1 and PDZ2 domains of GRASP65 participate in this molecular interaction. Mutagenesis experiments support these structural observations and demonstrate that they are required for GRASP65-GM130 association.

JNK2 controls fragmentation of the Golgi complex and the G2/M transition through phosphorylation of GRASP65

In mammalian cells, the Golgi complex is composed of stacks that are connected by membranous tubules. During G2, the Golgi complex is disassembled into isolated stacks. This process is required for entry into mitosis, indicating that the correct inheritance of the organelle is monitored by a 'Golgi mitotic checkpoint'. However, the regulation and the molecular mechanisms underlying this Golgi disassembly are still poorly understood. Here, we show that JNK2 has a crucial role in the G2-specific separation of the Golgi stacks through phosphorylation of Ser277 of the Golgi-stacking protein GRASP65 (also known as GORASP1). Inhibition of JNK2 by RNA interference or by treatment with three unrelated JNK inhibitors causes a potent and persistent cell cycle block in G2. JNK activity becomes dispensable for mitotic entry if the Golgi complex is disassembled by brefeldin A treatment or by GRASP65 depletion. Finally, measurement of the Golgi fluorescence recovery after photobleaching demonstrates that JNK is required for the cleavage of the tubules connecting Golgi stacks. Our findings reveal that a JNK2-GRASP65 signalling axis has a crucial role in coupling Golgi inheritance and G2/M transition.

CD83 and GRASP55 interact in human dendritic cells

CD83 is one of the best known surface markers for mature human dendritic cells (DCs). The full-length 45 kDa type-I membrane-bound form (mbCD83) is strongly glycosylated upon DCs maturation. As co-stimulatory properties of CD83 are attributed to mbCD83 surface expression is required for efficient T-cell stimulation by mature DCs. By yeast two-hybrid screening, we were able to identify GRASP55 as interaction partner of CD83. DCs maturation induces endogenous CD83 protein expression with simultaneous regulation of CD83 glycosylation, interaction and co-localization with GRASP55 and CD83 surface exposure. GRASP55 is especially known for its role in maintaining Golgi architecture, but also plays a role in Golgi transport of specific cargo proteins bearing a C-terminal valine residue. Here we additionally demonstrate that binding of CD83 and GRASP55 rely on the C-terminal TELV-motif of CD83. Mutation of this TELV-motif not only disrupted binding to GRASP55, but also altered the glycosylation pattern of CD83 and reduced its membrane expression. Here we show for the first time that GRASP55 interacts with CD83 shortly after induction of DC maturation and that this interaction plays a role in CD83 glycosylation as well as in surface expression of CD83 on DCs.

Self-organization of core Golgi material is independent of COPII-mediated endoplasmic reticulum export

The Golgi is a highly organized and dynamic organelle that receives and distributes material from and to the endoplasmic reticulum (ER) and the endocytic pathway. One open question about Golgi organization is whether it is solely based on ER-to-Golgi transport. Here, we analyzed the kinetics of Golgi breakdown in the absence of COPII-dependent ER export with high temporal and spatial resolution using quantitative fluorescence microscopy. We found that Golgi breakdown occurred in two phases. While Golgi enzymes continuously redistributed to the ER, we consistently observed extensive Golgi fragmentation at the beginning of the breakdown, followed by microtubule-dependent formation of a Golgi remnant structure (phase 1). Further Golgi disintegration occurred less uniformly (phase 2). Remarkably, cisternal Golgi morphology was lost early in phase 1 and Golgi fragments instead corresponded to variably sized vesicle clusters. These breakdown intermediates were devoid of COPI-dependent recycling material, but contained typical 'core' Golgi components. Furthermore, Golgi breakdown intermediates were able to disassemble and reassemble following cell division, indicating that they retained important regulatory capabilities. Taken together, these findings support the view that Golgi self-organization exists independently of ER-to-Golgi transport.

Proteome-Wide Effect of 17-β-Estradiol and Lipoxin A4 in an Endometriotic Epithelial Cell Line

Endometriosis affects approximately 10% of women of reproductive age. This chronic, gynecological inflammatory disease results in a decreased quality of life for patients, with the main symptoms including chronic pelvic pain and infertility. The steroid hormone 17-β Estradiol (E2) plays a key role in the pathology. Our previous studies showed that the anti-inflammatory lipid Lipoxin A4 (LXA4) acts as an estrogen receptor-alpha agonist in endometrial epithelial cells, inhibiting certain E2-mediated effects. LXA4 also prevents the progression of endometriosis in a mouse model via anti-proliferative mechanisms and by impacting mediators downstream of ER signaling. The aim of the present study was therefore to examine global proteomic changes evoked by E2 and LXA4 in endometriotic epithelial cells. E2 impacted a greater number of proteins in endometriotic epithelial cells than LXA4. Interestingly, the combination of E2 and LXA4 resulted in a reduced number of regulated proteins, with LXA4 mediating a suppressive effect on E2-mediated signaling. These proteins are involved in diverse pathways of relevance to endometriosis pathology and metabolism, including mRNA translation, growth, proliferation, proteolysis, and immune responses. In summary, this study sheds light on novel pathways involved in endometriosis pathology and further understanding of signaling pathways activated by estrogenic molecules in endometriotic epithelial cells.

The role of Aurora-A kinase in the Golgi-dependent control of mitotic entry

During mitosis, the Golgi complex undergoes a multi-step fragmentation process that is instrumental to its correct partitioning into the daughter cells. To prepare for this segregation, the Golgi ribbon is initially separated into individual stacks during the G2 phase of the cell cycle. Then, at the onset of mitosis, these individual stacks are further disassembled into dispersed fragments. Inhibition of this Golgi fragmentation step results in a block or delay of G2/M transition, depending on the experimental approach. Thus, correct segregation of the Golgi complex appears to be monitored by a 'Golgi mitotic checkpoint'. Using a microinjection-based approach, we recently identified the first target of the Golgi checkpoint, whereby a block of this Golgi fragmentation impairs recruitment of the mitotic kinase Aurora-A to, and its activation at, the centrosomes. Overexpression of Aurora-A can override this cell cycle block, indicating that Aurora-A is a major effector of the Golgi checkpoint. We have also shown that this block of Aurora-A recruitment to the centrosomes is not mediated by the known mechanisms of regulation of Aurora-A function. Here we discuss our findings in relation to the known functions of Aurora-A.

GRASP65 controls the cis Golgi integrity in vivo

GRASP65 and GRASP55 are peripheral Golgi proteins localized to cis and medial/trans cisternae, respectively. They are implicated in diverse aspects of protein transport and structure related to the Golgi complex, including the stacking of the Golgi stack and/or the linking of mammalian Golgi stacks into the Golgi ribbon. Using a mouse model, we interfered with GRASP65 by homologous recombination and confirmed its absence of expression. Surprisingly, the mice were healthy and fertile with no apparent defects in tissue, cellular or subcellular organization. Immortalized MEFs derived from the mice did not show any growth or morphological defects. However, despite the normal appearance of the Golgi ribbon, a fluorescence recovery after photobleaching assay revealed functional discontinuities specific to the cis cisternal membrane network. This leads to a strong change in the plasma membrane GSII lectin staining that was also observed in certain mutant tissues. These findings substantiate the role of GRASP65 in continuity of the cis Golgi network required for proper glycosylation, while showing that neither this continuity nor GRASP65 itself are essential for the viability of a complex organism.

The role of GRASPs in morphological alterations of Golgi apparatus: mechanisms and effects

The Golgi apparatus (GA) is a pivotal organelle in cell metabolism, functioning not only in the processing and transportation of cargoes but also in ion homeostasis, cell apoptosis, and stress sensing. We are interested in the intricate role of GA and the recently present novel concept of 'GA stress'. GA shows various morphological alterations in many neurodegenerative diseases and cell apoptosis induced by biochemical reagents, mechanisms in which oxidative stress is strongly involved. In turn, the structural changes and morphological alterations of the GA could also transduce stress signals. Therefore, besides the biochemical changes, more attention should be paid to the morphological alterations of the GA itself during pathological processes and diseases. The Golgi reassembly and stacking proteins (GRASPs) have been identified as important components acting in the transformation of Golgi structure, and they may thus affect the Golgi functions and cell behavior. In this review, we will discuss the intricate role of the GRASPs in remodeling the GA morphology and focus on their mechanisms and effects in the processes of Golgi stacking, mitosis, cell apoptosis, and cargo secretion. We would also like to provide a further prospective of their potential biological values in neurodegenerative diseases.

Membrane adhesion dictates Golgi stacking and cisternal morphology

Two classes of proteins that bind to each other and to Golgi membranes have been implicated in the adhesion of Golgi cisternae to each other to form their characteristic stacks: Golgi reassembly and stacking proteins 55 and 65 (GRASP55 and GRASP65) and Golgin of 45 kDa and Golgi matrix protein of 130 kDa. We report here that efficient stacking occurs in the absence of GRASP65/55 when either Golgin is overexpressed, as judged by quantitative electron microscopy. The Golgi stacks in these GRASP-deficient HeLa cells were normal both in morphology and in anterograde cargo transport. This suggests the simple hypothesis that the total amount of adhesive energy gluing cisternae dictates Golgi cisternal stacking, irrespective of which molecules mediate the adhesive process. In support of this hypothesis, we show that adding artificial adhesive energy between cisternae and mitochondria by dimerizing rapamycin-binding domain and FK506-binding protein domains that are attached to cisternal adhesive proteins allows mitochondria to invade the stack and even replace Golgi cisternae within a few hours. These results indicate that although Golgi stacking is a highly complicated process involving a large number of adhesive and regulatory proteins, the overriding principle of a Golgi stack assembly is likely to be quite simple. From this simplified perspective, we propose a model, based on cisternal adhesion and cisternal maturation as the two core principles, illustrating how the most ancient form of Golgi stacking might have occurred using only weak cisternal adhesive processes because of the differential between the rate of influx and outflux of membrane transport through the Golgi.

Isoform-specific tethering links the Golgi ribbon to maintain compartmentalization

Use of photoinactivation, cisternae-specific fluorescence recovery, and high-resolution microscopy shows that the membrane tethers GRASP65 and GRASP55 on early and late Golgi membranes, respectively, are critical to the specific, homotypic fusion of the membranes on which they reside.

Homotypic membrane tethering by the Golgi reassembly and stacking proteins (GRASPs) is required for the lateral linkage of mammalian Golgi ministacks into a ribbon-like membrane network. Although GRASP65 and GRASP55 are specifically localized to cis and medial/trans cisternae, respectively, it is unknown whether each GRASP mediates cisternae-specific tethering and whether such specificity is necessary for Golgi compartmentalization. Here each GRASP was tagged with KillerRed (KR), expressed in HeLa cells, and inhibited by 1-min exposure to light. Significantly, inactivation of either GRASP unlinked the Golgi ribbon, and the immediate effect of GRASP65-KR inactivation was a loss of cis- rather than trans-Golgi integrity, whereas inactivation of GRASP55-KR first affected the trans- and not the cis-Golgi. Thus each GRASP appears to play a direct and cisternae-specific role in linking ministacks into a continuous membrane network. To test the consequence of loss of cisternae-specific tethering, we generated Golgi membranes with a single GRASP on all cisternae. Remarkably, the membranes exhibited the full connectivity of wild-type Golgi ribbons but were decompartmentalized and defective in glycan processing. Thus the GRASP isoforms specifically link analogous cisternae to ensure Golgi compartmentalization and proper processing.

RNAi screening reveals a large signaling network controlling the Golgi apparatus in human cells

RNAi screening and automated image analysis reveal 180 kinases and phosphatases regulating the organization of the Golgi apparatus. Most of these genes also control the expression of specific glycans, pointing to a web of interactions between signaling cascades and glycosylation at the Golgi.

Golgi organization was probed with three markers of different Golgi compartments and quantitative morphological analysis.

Knockdowns of ∼20% of all known kinases and phosphatases affected the Golgi globally or in a compartment-specific manner, and were comparable in degree to the depletion of known membrane traffic regulators such as SNAREs.

Several cell surface receptors, their cognate ligands and downstream effectors regulate Golgi organization, suggesting a large regulatory network.

Most signaling genes affected both Golgi morphology and the expression of specific glycans.

The Golgi apparatus has many important physiological functions, including sorting of secretory cargo and biosynthesis of complex glycans. These functions depend on the intricate and compartmentalized organization of the Golgi apparatus. To investigate the mechanisms that regulate Golgi architecture, we developed a quantitative morphological assay using three different Golgi compartment markers and quantitative image analysis, and performed a kinome- and phosphatome-wide RNAi screen in HeLa cells. Depletion of 159 signaling genes, nearly 20% of genes assayed, induced strong and varied perturbations in Golgi morphology. Using bioinformatics data, a large regulatory network could be constructed. Specific subnetworks are involved in phosphoinositides regulation, acto-myosin dynamics and mitogen activated protein kinase signaling. Most gene depletion also affected Golgi functions, in particular glycan biosynthesis, suggesting that signaling cascades can control glycosylation directly at the Golgi level. Our results provide a genetic overview of the signaling pathways that control the Golgi apparatus in human cells.

Cell cycle regulation of Golgi membrane dynamics

The Golgi apparatus is a membranous organelle in the cell that plays essential roles in protein and lipid trafficking, sorting, processing, and modification. Its basic structure is a stack of closely aligned flattened cisternae. In mammalian cells, dozens of Golgi stacks are often laterally linked into a ribbon-like structure. Biogenesis of the Golgi during cell division occurs through a sophisticated disassembly and reassembly process that can be divided into three distinct but cooperative steps, including the deformation and reformation of the Golgi cisternae, stacks, and ribbon. Here, we review our current understanding of the protein machineries that control these three steps in the cycle of mammalian cell division: GRASP65 and GRASP55 in Golgi stack and ribbon formation; ubiquitin and AAA ATPases in postmitotic Golgi membrane fusion; and golgins and cytoskeleton in Golgi ribbon formation.

Signaling at the Golgi during mitosis

The Golgi complex of mammalian cells is composed of interconnected stacks of flattened cisternae that form a continuous membrane system in the pericentriolar region of the cell. At the onset of mitosis, this so-called Golgi ribbon is converted into small tubular-vesicular clusters in a tightly regulated fragmentation process, which leads to a temporary loss of the physical Golgi-centrosome proximity. Mitotic Golgi breakdown is required for Golgi partitioning into the two daughter cells, cell cycle progression and may contribute to the dispersal of Golgi-associated signaling molecules. Here, we review our current understanding of the mechanisms that control mitotic Golgi reorganization, its biological significance, and assays that are used to study this process.

Regulation of protein glycosylation and sorting by the Golgi matrix proteins GRASP55/65

The Golgi receives the entire output of newly synthesized cargo from the endoplasmic reticulum, processes it in the stack largely through modification of bound oligosaccharides, and sorts it in the trans-Golgi network. GRASP65 and GRASP55, two proteins localized to the Golgi stack and early secretory pathway, mediate processes including Golgi stacking, Golgi ribbon linking and unconventional secretion. Previously, we have shown that GRASP depletion in cells disrupts Golgi stack formation. Here we report that knockdown of the GRASP proteins, alone or combined, accelerates protein trafficking through the Golgi membranes but also has striking negative effects on protein glycosylation and sorting. These effects are not caused by Golgi ribbon unlinking, unconventional secretion or endoplasmic reticulum stress. We propose that GRASP55/65 are negative regulators of exocytic transport and that this slowdown helps to ensure more complete protein glycosylation in the Golgi stack and proper sorting at the trans-Golgi network.

MEK1 inactivates Myt1 to regulate Golgi membrane fragmentation and mitotic entry in mammalian cells

The pericentriolar stacks of Golgi cisternae are separated from each other in G2 and fragmented extensively during mitosis. MEK1 is required for Golgi fragmentation in G2 and for the entry of cells into mitosis. We now report that Myt1 mediates MEK1's effects on the Golgi complex. Knockdown of Myt1 by siRNA increased the efficiency of Golgi complex fragmentation by mitotic cytosol in permeabilized and intact HeLa cells. Myt1 knockdown eliminated the requirement of MEK1 in Golgi fragmentation and alleviated the delay in mitotic entry due to MEK1 inhibition. The phosphorylation of Myt1 by MEK1 requires another kinase but is independent of RSK, Plk, and CDK1. Altogether our findings reveal that Myt1 is inactivated by MEK1 mediated phosphorylation to fragment the Golgi complex in G2 and for the entry of cells into mitosis. It is known that Myt1 inactivation is required for CDK1 activation. Myt1 therefore is an important link by which MEK1 dependent fragmentation of the Golgi complex in G2 is connected to the CDK1 mediated breakdown of Golgi into tubules and vesicles in mitosis.

PKD controls mitotic Golgi complex fragmentation through a Raf-MEK1 pathway

Before entering mitosis, the stacks of the Golgi cisternae are separated from each other, and inhibiting this process delays entry of mammalian cells into mitosis. Protein kinase D (PKD) is known to be involved in Golgi-to-cell surface transport by controlling the biogenesis of specific transport carriers. Here we show that depletion of PKD1 and PKD2 proteins from HeLa cells by small interfering RNA leads to the accumulation of cells in the G2 phase of the cell cycle and prevents cells from entering mitosis. We further provide evidence that inhibition of PKD blocks mitotic Raf-1 and mitogen-activated protein kinase kinase (MEK) activation, and, as a consequence, mitotic Golgi fragmentation, which could be rescued by expression of active MEK1. Finally, Golgi fluorescence recovery after photobleaching analyses demonstrate that PKD is crucial for the cleavage of the noncompact zones of Golgi membranes in G2 phase. Our findings suggest that PKD controls interstack Golgi connections in a Raf-1/MEK1-dependent manner, a process required for entry of the cells into mitosis.

Sequential phosphorylation of GRASP65 during mitotic Golgi disassembly

GRASP65 phosphorylation during mitosis and dephosphorylation after mitosis are required for Golgi disassembly and reassembly during the cell cycle. At least eight phosphorylation sites on GRASP65 have been identified, but whether they are modified in a coordinated fashion during mitosis is so far unknown. In this study, we raised phospho-specific antibodies that recognize phosphorylated T220/T224, S277 and S376 residues of GRASP65, respectively. Biochemical analysis showed that cdc2 phosphorylates all three sites, while plk1 enhances the phosphorylation. Microscopic studies using these antibodies for double and triple labeling demonstrate sequential phosphorylation and dephosphorylation during the cell cycle. S277 and S376 are phosphorylated from late G2 phase through metaphase until telophase when the new Golgi is reassembled. T220/224 is not modified until prophase, but is highly modified from prometaphase to anaphase. In metaphase, phospho-T220/224 signal localizes on both Golgi haze and mitotic Golgi clusters that represent dispersed Golgi vesicles and Golgi remnants, respectively, while phospho-S277 and S376 labeling is more concentrated on mitotic Golgi clusters. Expression of a phosphorylation-resistant GRASP65 mutant T220A/T224A inhibited mitotic Golgi fragmentation to a much larger extent than the expression of the S277A and S376A mutants. In cytokinesis, T220/224 dephosphorylation occurs prior to that of S277, but after S376. This study provides evidence that GRASP65 is sequentially phosphorylated and dephosphorylated during mitosis at different sites to orchestrate Golgi disassembly and reassembly during cell division, with phosphorylation of the T220/224 site being most critical in the process.

Golgi complex fragmentation in G2/M transition: An organelle-based cell-cycle checkpoint

In mammalian cells, the Golgi complex is organized into a continuous membranous system known as the Golgi ribbon, which is formed by individual Golgi stacks that are laterally connected by tubular bridges. During mitosis, the Golgi ribbon undergoes extensive fragmentation through a multistage process that is required for its correct partitioning into the daughter cells. Importantly, inhibition of this Golgi disassembly results in cell-cycle arrest at the G2 stage, suggesting that accurate inheritance of the Golgi complex is monitored by a "Golgi mitotic checkpoint." Here, we discuss the mechanisms and regulation of the Golgi ribbon breakdown and briefly comment on how Golgi partitioning may inhibit G2/M transition.